[Unity初心者Tips]確実に！必要なComponentを入れるRequire ComponentとReset() ref: https://qiita.com/JunShimura/items/6576eb63c19c2c9f0c0a

TestClassA.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

[RequireComponent(typeof(TestClassB))]
[RequireComponent(typeof(TestClassC))]
[RequireComponent(typeof(Rigidbody))]
public class TestClassA : MonoBehaviour
{

    [SerializeField]
    TestClassB testClassB;
    [SerializeField]
    TestClassC testClassC;
    [SerializeField]
    Rigidbody rigidBody;
    [SerializeField]
    GameController gameController;
    [SerializeField]
    TestEnemy testEnemy;
    private void Reset()
    {
        testClassB = GetComponent<TestClassB>();
        testClassC = GetComponent<TestClassC>();
        rigidBody = GetComponent<Rigidbody>();
        gameController = FindObjectOfType<GameController>();
        if (gameController == null) {
            Debug.Log("ERROR!Gamecontroller not found");
        }
    } 
}